Lubricating oil composition for fluid dynamic bearing and hdd motor fabricated using the same

ABSTRACT

There are provided a lubricating oil composition for a fluid dynamic bearing, and a HDD motor fabricated using the same. The lubricating oil composition includes an ester base oil and 0.01 to 3.00 parts by weight of an additive relative to 100 parts by weight of the base oil and has a density of 0.900 g/cm 3  or less at 15° C. and a kinetic viscosity of 9.50 cSt or less at 40° C. According to the present invention, the HDD motor is fabricated by using a lubricating oil composition for a fluid dynamic bearing, the lubricating oil composition having advantages such as low viscosity at practical temperature, reduced evaporation loss and enhanced oxidation stability, thereby improving impact resistance and low temperature operational stability of a fluid dynamic motor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority of Korean Patent Application No. 10-2011-0140025 filed on Dec. 22, 2011, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a low density lubricating oil composition for a fluid dynamic bearing, enabling a fluid dynamic bearing motor to have superior impact resistance and operational stability at a low temperature, and a hard disk drive (HDD) motor fabricated using the same.

2. Description of the Related Art

A hard disk drive (HDD), as an information storage device, uses a read/write head to read data stored in a disk or record (write) data to the disk.

Such a HDD requires a disk driver to drive the disk and the disk driver may be provided with a small spindle motor.

The small spindle motor may include a fluid dynamic bearing assembly having lubricating fluid interposed between a shaft and a sleeve thereof, in order to support the shaft using fluid pressure generated in the lubricating fluid.

In a case in which viscosity of the lubricating fluid is increased at a low temperature during the rotation of the spindle motor, viscosity resistance of the lubricating fluid against a dynamic pressure generating groove formed during the rotation of the motor may be higher. Consequently, power loss may be incurred in the motor.

On the contrary, when the lubricating fluid undergoes respective decreases in thermal expansion and viscosity during the rotation of the motor in a high temperature range, the lubricating fluid does not sufficiently serve as a support.

For such reasons, the lubricating fluid requires contrary viscosity behavior characteristics, in which it maintains a relatively low viscosity in a low temperature range while the viscosity is not decreased in a high temperature range.

In order to satisfy the foregoing viscosity characteristics, for example, adding additive substance(s) such as an antioxidant, an extreme pressure additive or the like to a base oil including a specific ester compound as a main component have been developed and further research with regard thereto is currently being undertaken.

However, even in the case that the lubricating fluid including the foregoing various additives exhibits initial positive effects, while the lubricating fluid is used for an extended period time, the lubricating fluid may be evaporated and viscosity characteristics thereof may be varied, thus causing difficulties in retaining desired effects.

Meanwhile, with a growing trend towards the compactness, high precision, high rotational speed and low power dissipation of the motor, the lubricating fluid needs thermal resistance, oxidation stability, low evaporation, abrasion resistance, and the like.

Furthermore, there is a need for a lubricating fluid capable of improving the impact resistance of a fluid dynamic motor.

RELATED ART DOCUMENT

(Patent Document 1) Japanese Patent Laid-Open Publication No. 2011-111463

SUMMARY OF THE INVENTION

An aspect of the present invention provides a low density lubricating oil composition for a fluid dynamic bearing, enabling a fluid dynamic bearing motor to have superior impact resistance and operational stability at a low temperature, and a hard disk drive (HDD) motor fabricated using the same.

According to an aspect of the present invention, there is provided a lubricating oil composition for a fluid dynamic bearing, including: an ester base oil; and 0.01 to 3.00 parts by weight of an additive with respect to 100 parts by weight of the base oil, wherein the lubricating oil composition has a density of 0.900 g/cm or less at 15° C. and a kinetic viscosity of 9.50 cSt or less at 40° C.

The density of the lubricating oil composition may range from 0.866 g/cm³ to 0.900 g/cm³ at 15° C.

The base oil may have a viscosity of 2.30 cP or less at 100° C.

The additive may include at least one selected from the group consisting of an antioxidant, an abrasion inhibitor, a corrosion inhibitor, an extreme pressure additive, a viscosity modifier, an antistatic agent and a deactivating agent.

The additive may include 2,2′-methylene-bis(4-methyl-6-tert-butylphenol) as an antioxidant.

The additive may include barium diphenylamine-4-sulfonate as a metal antioxidant.

The additive may include tricresyl phosphate as an internal pressure inhibitor.

According to another aspect of the present invention, there is provided a HDD motor including a lubricating oil composition for a fluid dynamic bearing, the lubricating oil composition including an ester base oil and 0.01 to 3.00 parts by weight of an additive to 100 parts by weight of the base oil and having a density of 0.900 g/cm³ or less at 15° C. and a kinetic viscosity of 9.50 cSt or less at 40° C.

The density of the lubricating oil composition may range from 0.866 g/cm³ to 0.900 g/cm³ at 15° C.

The base oil may have a viscosity of 2.30 cP or less at 100° C.

The additive may include at least one selected from the group consisting of an antioxidant, an abrasion inhibitor, a corrosion inhibitor, an extreme pressure additive, a viscosity modifier, an antistatic agent and a deactivating agent.

The additive may include 2,2′-methylene-bis(4-methyl-6-tert-butylphenol) as an antioxidant.

The additive may include barium diphenylamine-4-sulfonate as a metal antioxidant.

The additive may include tricresyl phosphate as an internal pressure inhibitor.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a schematic cross-sectional view illustrating a hard disk drive (HDD) motor having a fluid dynamic bearing assembly according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the shapes and dimensions of components may be exaggerated for clarity, and the same reference numerals will be used throughout to designate the same or like components.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

A lubricating oil composition for a fluid dynamic bearing according to an embodiment of the present invention includes: an ester base oil; and 0.01 to 3.00 parts by weight of an additive with respect to 100 parts by weight of the base oil, wherein the lubricating oil composition has a density of 0.900 g/cm³ or less at 15° C. and a kinetic viscosity of 9.50 cSt or less at 40° C.

A detailed description thereof will be provided below.

The lubricating oil composition for a fluid dynamic bearing may include an ester base oil.

The base oil is not particularly limited so long as it maybe usable for a fluid dynamic bearing. For example, an ester based compound may be used therefor.

According to the embodiment of the invention, the lubricating oil composition for a fluid dynamic bearing may include 0.01 to 3.00 parts by weight of an additive with respect to 100 parts by weight of the base oil.

The additive may include at least one selected from the group consisting of an antioxidant, an abrasion inhibitor, a corrosion inhibitor, an extreme pressure additive, a viscosity modifier, an antistatic agent and a deactivating agent, without being limited thereto. That is, a variety of additives may be used.

A very small amount of additive may be added to the lubricating oil for a fluid dynamic bearing, thereby improving a long-term high temperature reliability of the lubricating oil.

The additive may include an antioxidant, such as 2,2′-methylene-bis(4-methyl-6-tert-butylphenol), without being limited thereto.

The additive may further include a metal antioxidant, such as barium diphenylamine-4-sulfonate, without being limited thereto.

Further, the additive may include an internal pressure inhibitor, such as tricresyl phosphate, without being limited thereto.

0.01 to 3.00 parts by weight of the additive may be contained with respect to 100 parts by weight of the base oil, thereby allowing the lubricating oil to maintain a low viscosity in a low temperature range without a reduction of the viscosity in a high temperature range.

In a case in which the content of the additive is less than 0.01 parts by weight with respect to 100 parts by weight of the base oil, the effects of the additive may not be sufficiently achieved.

On the other hand, in a case in which the content of the additive exceeds 3.00 parts by weight with respect to 100 parts by weight of the base oil, the lubricating oil for a fluid dynamic bearing may encounter deterioration in physical properties.

Meanwhile, the lubricating oil composition for a fluid dynamic bearing according to the embodiment of the invention may have a density of 0.900 g/cm or less at 15° C. and a kinetic viscosity of 9.50 cSt or less at 40° C.

According to the embodiment of the invention, the density of the lubricating oil composition for a fluid dynamic bearing may be adjusted to 0.900 g/cm or less at 15° C., thereby improving impact resistance of the fluid dynamic bearing motor and operational stability thereof at a low temperature.

Although selection of the lubricating oil is usually based on kinetic viscosity, the driving of the fluid dynamic bearing motor is practically associated with dynamic viscosity (hereinafter, also referred to as ‘viscosity’).

In this regard, since the dynamic viscosity is represented as a function of kinetic viscosity x density, when the lubricating oil has a low density, the dynamic viscosity thereof may be decreased, in particular, such effects may be more remarkable in a low temperature range in which the viscosity is rapidly increased.

That is, the density of the lubricating oil composition for a fluid dynamic bearing is adjusted to 0.900 g/cm³ or less at 15° C., such that the lubricating oil composition may maintain a low level of viscosity in a low temperature range to thereby improve low temperature operational stability.

In addition, the use of the low density lubricating oil may allow for improvements in the impact resistance of the fluid dynamic bearing motor.

In the case in which the lubricating oil composition for a fluid dynamic bearing has a density of more than 0.900 g/cm³ at 15° C., it may be problematic in terms of low temperature operational stability and/or impact resistance of the fluid dynamic bearing motor.

Meanwhile, according to the embodiment of the invention, the kinetic viscosity of the lubricating oil composition for a fluid dynamic bearing at 40° C. may be adjusted to 9.50 cSt or less, thereby improving impact resistance of the fluid dynamic bearing motor as well as low temperature operational stability.

In other words, the density of the lubricating oil composition for a fluid dynamic bearing is adjusted to be 0.900 g/cm or less at 15° C. while the kinetic viscosity thereof is adjusted to be 9.50 cSt or less at 40° C., whereby the dynamic viscosity thereof may be reduced and excellent low temperature operational stability may be attained.

In the case in which the kinetic viscosity of the lubricating oil composition for a fluid dynamic bearing exceeds 9.50 cSt at 40° C., the dynamic viscosity of the lubricating oil composition cannot be maintained to a low level even when the density of the lubricating oil composition is adjusted to be 0.900 g/cm³ or less, thereby causing problems in terms of low temperature operational stability and impact resistance of the fluid dynamic bearing motor.

The kinetic viscosity of the lubricating oil composition for a fluid dynamic bearing according to the embodiment of the invention may be measured at 0° C., 25° C., 40° C. and 100° C.

The viscosity may be measured using a viscometer, i.e., Brookfield DB-III Rheometer. In order to determine temperature-dependent tendency, the measurement may be performed in four different temperature ranges (0° C., 25° C., 40° C., and 100° C.), respectively.

Such lubricating oil composition for a fluid dynamic bearing is not particularly limited and, for example, may be suitably used for a fluid bearing of a hard disk drive (HDD) motor.

In the case of a small HDD, reduced power consumption, low temperature operational stability and impact resistance may be very important factors.

The lubricating oil composition according to the embodiment of the invention shows a reduction in friction loss and improvement in low temperature operational stability, thereby satisfying the above-described desired conditions of the small HDD.

FIG. 1 is a schematic cross-sectional view illustrating an HDD motor having a fluid dynamic bearing assembly according to an embodiment of the present invention.

Referring to FIG. 1, the HDD motor according to this embodiment of the present invention may include a lubricating oil composition for a fluid dynamic bearing including an ester base oil, and 0.01 to 3.0 parts by weight of an additive with respect to 100 parts by weight of the base oil, wherein the lubricating oil composition has a density of 0.900 g/cm³ or less at 15° C. and a kinetic viscosity of 9.50 cSt or less at 40° C.

Hereinafter, the HDD motor according to the embodiment of the present invention will be described in detail, except that the description overlapping with that of the foregoing embodiment of the present invention will be omitted.

A motor 10 having a base assembly for a motor (hereinafter, referred to as ‘base assembly’) may include a base assembly 100 having a base 110 for a motor (hereinafter, referred to as ‘base’), a sleeve 220 supporting the rotation of a rotational member, and a core 240 having a coil 230 wound thereon.

First, terms regarding directions are defined as follows: as shown FIG. 1, an axial direction refers to a vertical direction with respect to the shaft 210, and an outer or inner radial direction refers to a direction towards an outer edge of a hub 250 with respect to the shaft 210 or a direction towards the center of the shaft 210 with respect to the outer edge of the hub 250.

In addition, a circumferential direction refers to a rotational direction of the shaft 210, that is, a direction rotating along an outer circumferential surface of the shaft 210.

The base assembly 100 may include the base 110 and a pull plate 120, and the base 110 may be coupled to the core 240 having the coil 230 wound therearound.

In other words, the base 110 may be a fixed member supporting a rotational member including the hub 250, and may be coupled to the core 240 having the coil 230 wound therearound and the coil 230 generating electromagnetic force having a predetermined magnitude when power is applied thereto.

Here, the base 110 may have a protrusion 112 and a body part 114, and an inner circumferential surface of the protrusion 112 may be bonded to an outer circumferential surface of the sleeve 220 supporting the shaft 210, thereby supporting the sleeve 220.

That is, the protrusion 112 may have a hollow and be protruded upward in the axial direction, and the sleeve 220 supporting the shaft 210 may be inserted into the hollow and they are combined by welding, bonding and/or press-fitting.

In addition, an outer circumferential surface of the protrusion 112 may be bonded to the core 240 having the coil 230 wound therearound. In order to ensure desired rotational stability of the motor 10, rigidity should be secured.

In this regard, the body part 114 of the base 110 may be combined with the pull plate 120, and the pull plate 120 may prevent the excessive floating of the rotational member including the shaft 210 and the hub 250.

More particularly, the pull plate 120 may be combined with the body part 114 corresponding to a bottom surface of a magnet 260 coupled to the hub 250by a bonding method or the like, and may have magnetic properties such that it allows for magnetic attraction to the magnet 260.

The shaft 210 and the hub 250 of the motor 10 according to the embodiment of the invention need to be floated to a predetermined height for stable rotation. However, in the case of the excessive floating thereof beyond the predetermined height, the performance of the rotational members may be adversely affected thereby.

In this case, the base 112 may be combined with the pull plate 120 in order to prevent the excessive floating of the shaft 210 and the hub 250, and magnetic attraction between the pull plate 120 and the magnet 260 may prevent the excessive floating of the foregoing rotational members.

The shaft 210 may be a rotational member coupled to the hub 250 and rotating together with the hub 250, and may be supported by the sleeve 220.

The sleeve 220 supports the rotation of the rotational members, that is, the shaft 210 and the hub 250. More particularly, the sleeve 220 may support the shaft 210 while allowing a top end of the shaft 210 to be protruded upward in the axial direction, and may be fabricated by forging Cu or Al, or sintering Cu-Fe alloy powder or SUS powder.

In addition, the sleeve 220 may have a shaft hole, into which the shaft 210 is inserted with a micro clearance therebetween. The micro clearance is filled with oil O such that the shaft 210 may be stably supported by radial dynamic pressure through the oil O.

The hub 250 is a rotational structure provided to rotate with respect to a fixed structure including the base 110, and may have the ring-shaped magnet 260 facing the core 240 with a predetermined interval therebetween.

Here, the rotational driving force of the motor 10 may be generated by interaction between the magnet 260 and the coil 230 wound around the core 240.

A HDD motor according to another embodiment of the present invention includes a lubricating oil composition 170 for a fluid dynamic bearing, such that it may efficiently reduce friction loss of the motor while having low viscosity and attain superior operational stability at a low temperature and excellent impact resistance.

Furthermore, since the HDD motor is fabricated using the lubricating oil composition for a fluid dynamic bearing having reduced viscosity at practical temperature, low evaporation loss and improved oxidation stability, the reliability thereof when being used for a long time may be enhanced.

A method of fabricating the HDD motor 10 may be substantially identical to a general method, except that the lubricating oil composition 170 for a fluid dynamic bearing is used therein.

Hereinafter, the following examples will be given for concretely describing the present invention; however, the present invention is not limited thereto.

EXAMPLES 1 TO 3

In Example 1, a base oil was prepared by mixing dioctyl adipate and 2-hexyldecyl dodecanoate in a ratio of 1:1. The base oil was used in an amount of 97 parts by weight.

The dioctyl adipate and the 2-hexyldecyl dodecanoate were represented by Formula 1 and 2, respectively. Here, the dioctyl adipate had a density of 0.927 g/cm³ at 15° C., while the 2-hexyldecyl dodecanoate had a density of 0.859 g/cm³ at 15° C.

Then, at least one additive selected from the group consisting of an antioxidant, an abrasion inhibitor, a corrosion inhibitor, an extreme pressure additive, a viscosity modifier, an antistatic agent and a deactivating agent was mixed in a ratio of 3 parts by weight with respect to total weight, thereby forming a lubricating oil for a fluid dynamic bearing.

The lubricating oil exhibited a density of 0.895 g/cm³ at 15° C.

In Example 2, a base oil was prepared by using ethylhexyl oleate in an amount of 97 parts by weight.

The ethylhexyl oleate was represented by Formula 3 and had a density of 0.866 g/cm³ at 15° C.

Also, the lubricating oil for a fluid dynamic bearing was prepared by mixing the same additive as that used in Example 1 in a ratio of 3 parts by weight with respect to total weight. The prepared lubricating oil had a density of 0.870 g/cm³ at 15° C.

In Example 3, a base oil was prepared by using 2-hexyldecyl dodecanoate having a low density in an amount of 97 parts by weight.

The 2-hexyldexyl dodecanoate may be represented by

Formula 4 and had a density of 0.859 g/cm³ at 15° C.

Also, the lubricating oil for a fluid dynamic bearing was prepared by mixing the same additive as that used in Example 1 in a ratio of 3 parts by weight with respect to total weight.

The prepared lubricating oil had a density of 0.866 g/cm³ at 15° C.

COMPARATIVE EXAMPLES 1 AND 2

In Comparative Example 1, a base oil was prepared by using a neopentylglycol based ester lubricant in an amount of 97 parts by weight.

The neopentylglycol based ester lubricant was represented by Formula 5 and had a density of 0.939 g/cm³ at 15° C.

The types and contents of additives were substantially identical to those described in the foregoing examples. The prepared lubricating oil for a fluid dynamic bearing had a density of 0.944 g/cm³ at 15° C.

In Comparative Example 2, a base oil was prepared by using 3-methyl-1,5-pentandiol based ester lubricant in an amount of 97 parts by weight.

The 3-methyl-1,5-pentadiol based ester lubricant was represented by Formula 6 and had a density of 0.928 g/cm³ at 15° C.

The types and contents of additives were substantially identical to those described in the foregoing examples. The prepared lubricating oil for a fluid dynamic bearing had a density of 0.944 g/cm³ at 15° C.

The following Table 1 illustrates densities, kinetic viscosities, (dynamic) viscosities of lubricating oil compositions prepared in the Examples and the Comparative Examples, as well as comparison between test results of low temperature operation and impact resistance of the respective lubricating oil compositions.

More particularly, the viscosity was measured using a viscometer, i.e., Brookfield DB-III Rheometer. In order to assess temperature-dependent tendency, respective components were measured in four different temperature ranges (0° C., 25° C., 40° C. and 100° C.)

The low temperature operation test was executed with reference to 0° C., and results thereof are shown in Table 1 below.

Impact resistance test was performed by applying severe impact ranging from 350 to 1000 times of a gravitational constant according to test methods and comparing results thereof to determine whether features of the lubricating oil were altered or not.

TABLE 1 Items Com. Ex. 1 Com. Ex. 2 Ex. 1 Ex. 2 Ex. 3 Density (g/cm³) 0.944 0.934 0.895 0.870 0.866 Kinetic 100° C. 2.57 2.54 2.47 2.58 2.66 viscosity  40° C. 9.56 9.54 9.04 9.47 9.17 (cSt)  25° C. 14.90 14.79 15.40 15.34 15.36  0° C. 48.60 47.50 51.30 49.20 51.00 Dynamic 100° C. 2.43 2.37 2.21 2.24 2.30 viscosity  40° C. 9.02 8.91 8.09 8.24 7.94 (cP)  25° C. 14.07 13.81 13.78 13.35 13.30  0° C. 45.88 44.37 45.91 42.80 44.17 Low temperature  0° C. 115 112 116 107 110 operation (mA) Failure in impact — 16.7 13.3 3.3 0 0 resistance test (%)

Referring to Table 1, it can be seen that the lubricating oil compositions according to Examples 1 to 3 of the present invention exhibited superior low temperature operation and, as a result of testing impact resistance of a motor, failures were considerably reduced. Therefore, it was demonstrated that impact resistance was efficiently improved.

On the contrary, in the case of Comparative Examples 1 and 2, the prepared compositions exhibited a density of more than 0.900 g/cm³ at 15° C. and a kinetic viscosity of more than 9.50 cSt at 40° C. and, according to test results, it was found that the foregoing compositions had problems in terms of low temperature operation and impact resistance.

As set forth above, according to embodiments of the present invention, a HDD motor is fabricated to include a lubricating oil composition for a fluid dynamic bearing having advantageous features such as low viscosity at practical temperature, reduced evaporation loss and improved oxidation stability, such that impact resistance and low temperature operational stability of the fluid dynamic bearing motor may be enhanced.

While the present invention has been shown and described in connection with the embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims. 

What is claimed is:
 1. A lubricating oil composition for a fluid dynamic bearing, comprising: an ester base oil; and 0.01 to 3.00 parts by weight of an additive with respect to 100 parts by weight of the base oil, wherein the lubricating oil composition has a density of 0.900 g/cm or less at 15° C. and a kinetic viscosity of 9.50 cSt or less at 40° C.
 2. The lubricating oil composition of claim 1, wherein the density of the lubricating oil composition ranges from 0.866 g/cm³ to 0.900 g/cm³ at 15° C.
 3. The lubricating oil composition of claim 1, wherein the base oil has a viscosity of 2.30 cP or less at 100° C.
 4. The lubricating oil composition of claim 1, wherein the additive includes at least one selected from the group consisting of an antioxidant, an abrasion inhibitor, a corrosion inhibitor, an extreme pressure additive, a viscosity modifier, an antistatic agent and a deactivating agent.
 5. The lubricating oil composition of claim 1, wherein the additive includes 2,2′-methylene-bis(4-methyl-6-tert-butylphenol) as an antioxidant.
 6. The lubricating oil composition of claim 1, wherein the additive includes barium diphenylamine-4-sulfonate as a metal antioxidant.
 7. The lubricating oil composition of claim 1, wherein the additive includes tricresyl phosphate as an internal pressure inhibitor.
 8. A HDD motor including a lubricating oil composition for a fluid dynamic bearing, the lubricating oil composition including an ester base oil and 0.01 to 3.00 parts by weight of an additive with respect to 100 parts by weight of the base oil and having a density of 0.900 g/cm or less at 15° C. and a kinetic viscosity of 9.50 cSt or less at 40° C.
 9. The HDD motor of claim 8, wherein the density of the lubricating oil composition ranges from 0.866 g/cm³ to 0.900 g/cm³ at 15° C.
 10. The HDD motor of claim 8, wherein the base oil has a viscosity of 2.30 cP or less at 100° C.
 11. The HDD motor of claim 8, wherein the additive includes at least one selected from the group consisting of an antioxidant, an abrasion inhibitor, a corrosion inhibitor, an extreme pressure additive, a viscosity modifier, an antistatic agent and a deactivating agent.
 12. The HDD motor of claim 8, wherein the additive includes 2,2′-methylene-bis(4-methyl-6-tert-butylphenol) as an antioxidant.
 13. The HDD motor of claim 8, wherein the additive includes barium diphenylamine-4-sulfonate as a metal antioxidant.
 14. The HDD motor of claim 8, wherein the additive includes tricresyl phosphate as an internal pressure inhibitor. 